In this competitive renewal, we are investigating the molecular mechanisms of signaling pathways mediated by apoE receptors in the brain. In previous studies, we have identified several neuronal processes that are altered by apoE interacting with its1 receptors, including alteration of NMDA receptor function, phosphorylation of the Disabled-1 protein, effects on kinases (activation of ERK and inactivation of JNK), and proteolytic processing of apoE receptors themselves. In this proposal, we will investigate the connections between the molecules necessary for these functions in vitro and in vivo. Understanding the basic biology of apoE in the brain will help in understanding the effects of apoE isoforms on the risk of Alzheimer's disease. In Aim 1, we will define the molecular connection between apoE receptors and NMDA receptors. Several groups have now defined that apoE receptors affect calcium influx via NMDA receptors and long term potentiation. We hypothesize that apoE receptors interact with NMDA receptors via both extracellular and intracellular domains. We will define the domains of apoE receptors and the domains of the NMDA receptor subtypes that are important for these interactions, test whether the adaptor protein PSD-95 is important for the interactions, and determine what effects these interactions have on the electrophysiology of NMDA receptors. In aim 2, we will identify the effects of cytoplasmic adaptor proteins on intracellular signaling cascades stimulated by apoE in neurons and glia. We have identified several signaling cascades in neurons that depend on different apoE receptor adaptor proteins. We hypothesize that some of these cascades also function in glia. We will also determine whether apoE binding to receptors can alter Dab1 phosphorylation through receptor clustering, bringing it in contact with Fyn kinase, via interactions with PSD-95. We will test whether inhibition of JNK requires cleavage of the C-terminus of apoE receptors, and release of JNK- interacting proteins from membrane compartments. Finally, we will determine whether the effect of apoE on inhibiting glial inflammation is transduced via one of these signaling cascades. In aim 3, we will determine which signaling cascades identified in vitro are active in vivo. ApoE is induced acutely after several types of brain damage, and is present in AD chronically as a component of senile plaques. We will examine how apoE affects processes in vivo using three systems: injection of apoE into the brain parenchyma to examine the acute effects on signaling cascades in neurons and glia; analysis of apoE-labeled fluorescent beads to test the internalization of apoE into cell types in vivo; and analysis of mice transgenic for the three APOE isoforms to test the chronic effects of apoE on NMDA receptor subtypes.